studioair I don't really understand where my HTML, CSS skills come into play?
You do not need any coding skills when using WordPress or other CMS at basic level. Just add images and text. Done. I guess that Squarespace is another CMS like WIX. A CMS locked to a certain web hosting company as I understand it. Harder to move to another vendor. More about this...
What I have learned using both Drupal and WordPress is that any CMS is a "language" of its own. You have to learn the CMS separately. Basically you work with "plugins" to do "programming" stuff.
I also don't understand how far you can customize these templates?
You can customize these templates a lot. By using your HTML, CSS skills.
Can you build a website from scratch on this platform?
42.9% of the world's websites are built with WordPress according to w3techs.com. So I guess that the answer is YES! You can look at the CMS market share here.
But some claims that static sites are beginning to be more popular. Cannot confirm this, but I use static sites myself today. I got tired of the limitations and all the extra dead meat when I used CMS. The price you have to pay for the CMS simplicity IMO is slower site because of all extra stuff you do not use.